1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a laser beam printer, and more particularly, to an image forming apparatus using a two-component developer.
2. Related Background Art
Heretofore, in general, in an image forming apparatus of an electrophotographic or an electrostatic recording system such as a copying machine which uses a two-component developer mainly composed of a toner and a carrier, when the toner is consumed and a developer density inside an developing apparatus, that is, a mixture ratio of the toner to the carrier (hereinafter referred to as “TD ratio”) changes, so that an image of a low image ratio is continuously reproduced, the imbedding of an external additive externally added for giving fluidity to the toner as well as adherence of the toner and the external additive to the carrier surface have often changed developing property and transferring property, thereby remarkably deteriorating these properties.
Hence, to maintain the TD ratio which is a toner density in the developer and the density of a formed image constant, a technique for accurately detecting this TD and the image density becomes important.
The image forming apparatus adapting the two-component developer system is provided with a replenishment developer container (replenishment container) connected to a developer container for replenishing a consumed toner included in the developer inside the developer container which is a developer containing portion of a developing apparatus. The replenishment developer container contains a replenishment toner as a developer cartridge, and is usually detachably attachable so as to be replaced when an inner toner is consumed. A toner replenishing control is proposed and put to a practical use, which decides a toner amount to be replenished to the developing apparatus by the replenishment container so that a detection value by an image density detection and the TD ratio detection of the developer inside the developer container (hereinafter referred to as “developer density detection”) is always maintained constant.
In other words, according to the above described toner replenishing control, a lowering of toner charge amount over a long period of time during image formation is avoided by adequately lowering the toner density of the two component developer of the developing container, and the image density during image formation is maintained constant over a long period of time.
The toner replenishing control is executed at a pre-rotation time which is a non image time of the image formation, and when an image density lowering is detected by image density detecting means (image density sensor) in an density detection developed image (patch), the toner of the developer inside the developing apparatus is replenished. In case a replenishing amount from the replenishment container to the developing apparatus based on this image density detection result becomes excessive, a signal of the replenishing amount to be excessive is sent out by developer density detection inside the developing apparatus, and a control of replenishing amount as well as a control of stopping the replenishment are performed.
In general, the developer density detection is performed by developer density detecting means such as a light sensor or a magnetic permeability detection sensor and the like provided in the developing apparatus where the developer is contained. In the case of the light sensor, by irradiating a light, the carrier is attracted because of its ordinary black color, and the TD ratio is discriminated by detecting a change in the toner amount, that is, the reflected light amount corresponding to the TD ratio. The magnetic permeability detection sensor discriminates the TD ratio by detecting an apparent magnetic permeability change in the developer, which lowers when the TD ratio rises.
In the meantime, the detection of the image density which is a toner bearing amount in the image portion of an image to be formed, in the case of the image forming apparatus of the electrophotographic system already charged, forms a density detection electrostatic latent image (patch latent image) by a predetermined latent image contrast on an image bearing member such as a drum-shaped electrophotographic photosensitive member (photosensitive drum), and develops this patch latent image by the two-component developer contained in the developing apparatus, and takes it as the density detection developed image (patch). On this patch image, a light is irradiated by the light sensor as the image density sensor, and the detection is performed depending on various magnitudes of the reflected light.
While the electrostatic latent image is formed by changing a surface potential in the image portion of the uniformly charged image bearing member, the difference of the surface potential between the non-image portion and the image portion is the latent image contrast. In the image forming apparatus of the electrophotographic system, since the image bearing member is a photosensitive drum, in the toner replenishing control, the charged potential and the exposed light amount are adjusted so as to become a predetermined latent image contrast on this photosensitive drum, thereby forming a patch latent image.
Incidentally, at the installing time of the image forming apparatus and at the replacing time of the developer, though a new developer is filled in the developing apparatus, the developer right after being transferred to the developing apparatus inside the image forming apparatus from a container to be used and hermetically sealed at the time of shipment is in a state of being moisture conditioned to a moisture amount inside the container before the transfer, and the developer is gradually exposed to the outside air inside the developing apparatus, and is moisture conditioned to the moisture amount detected by environmental detecting means (environmental sensor) provided inside the image forming apparatus. During this moisture conditioning, a tribo-electrostatic charge (tribo) in the developer also is changed from a value inside the replenishment container to a value corresponding to the moisture amount detected by the environmental sensor.
Here, to correspond to the change of endurance and environment, a patch is periodically formed by a predetermined density, and by detecting an output value of the image density, a density signal from the apparatus control portion is corrected, and based on that information, the latent image contrast, which is a latent image forming condition to form a patch latent image by the toner replenishing control, is changed. In this manner, the toner replenishing control can be performed by the latent image contrast corresponding to the environment at that time. Further, at this time, a gradation control is also executed, and by finding the latent image contrast in each gradation, a control to maintain a desired gradation characteristic can be performed.
Here, when a periodic control for deciding the latent image contrast according to this environment is performed by the patch of the maximum image density in the predetermined density, that is, the maximum image density developed image, a scope of the detection density is broadened out and becomes preferable, and thus, the periodic control is often executed as a so-called maximum image density control (Dmax control). Since the Dmax control takes a time to execute in a pre-rotation for every image formation, at the rising time of the apparatus and after having risen the apparatus, the Dmax control is executed at periodically determined intervals depending on the number of image formation sheets and time.
That is, at the drastically changed time of an apparatus environment or at the initial density setting time by the developer inside the developing apparatus or at the rise time of the apparatus after replenishment or replacement of the developer from the outside, the-maximum image density control is executed, so that the latent image contrast set up by such maximum image density control according to the detection value of the environmental sensor by the subsequent toner replenishing control is used and controlled so as to become a reference density for the entire environment.
However, in recent years, because of a trend toward a high image quality, the toner and carrier inside the developer come to become small in diameter, and a superficial area as the developer becomes large, and a moisture conditioning time of the developer tends to be long.
Hence, in case the installing environment of the apparatus drastically changes, the developer is not sufficiently moisture-conditioned, and before the tribo becomes a value corresponding to a moisture amount detected by the environmental sensor, a so-called environmental mismatch state, in which a patch image detection timing in the maximum image density control is invited, occurs. That is, in this maximum image density control, a situation where the environment and the latent image are set in a mismatched state occurs. Particularly, at the installing time of the image forming apparatus, and at the initial setting time of the patch image detection right after the replacement of the developer, this environmental mismatch state tends to occur.
To be specific, when the developer is filled in the developing container of the developing apparatus at the setting time of the apparatus and at the replacing time of the developer, and the like, a moisture amount (water mass contained in the air of 1 m3) inside the developing container depends on the environment at the filling time, and in general, the amount fluctuates from 1 g to 20 g. In the meantime, similarly, since the inside of the replenishment container installed in the image forming apparatus is hermetically sealed, the moisture amount inside the container is hardly changed for a long period of time.
For example, in a state in which the moisture amount inside the replenishment container is 20 g, and in the initial period, when the toner is filled in an empty developer container from this container, in case the detection moisture amount of the environmental sensor is 1 g, though the latent image contrast is outputted at a value corresponding to the moisture amount of 1 g which is the detection value of the environmental sensor, since an actual frictional charging amount of the developer is a value corresponding to the moisture amount of 20 g inside the replenishment container, a phenomenon that the patch image density is sharply deviated from the reference density occurs.
As a result, the initial density setting is incapacitated, and even when the density setting is attempted, since it is deviated from the reference density, it is small in sensitivity, and a desired image density is not achieved by the latent image contrast set at the maximum image density control time.
Hence, in the course of the developer being moisture conditioned, a problem arises because of a non-moisture conditioned developer. For example, the patch density is detected thin by the change of the tribo, and the toner density of the developer is set high, so that the problems such as a fogging, a toner flying, and in the worst case, a spilling out of the toner from the developing apparatus are brought about.
Hence, in Japanese Patent Application Laid-Open No. H10-83115, a proposal is made in which a state quantity regarding developing characteristics, for example, the maximum toner adhering amount capable of image formation, a developing starting voltage, and the like are calculated, so that at least one of the processing such as agitation of the developer of the developing means, the toner replenishment, and the toner consumption is executed, thereby executing a developer aging so as to adjust a developer state.
However, in the developer aging executing time, every time the image formation is executed, the toner density is required to be changed until it becomes a desired developing characteristic, and further, for such setting, it requires a long period of time, so that there arises a problem of the rising time becoming long.
Further, in Japanese Patent Application Laid-Open No. 2001-194837, a proposal is made in which, at the initial density setting time of the patch image, regardless of the moisture amount detected by the environmental sensor, the patch image is formed by developing the patch latent image formed by the predetermined latent image contrast, and based on this image density, an image density output setting is made. In this method, there is outwardly no problem of the developer being non-moisture conditioned.
However, according to this method, though a patch image is formed by a constant latent image contrast, the predetermined value thereof is set regardless of a degree of the moisture conditioning in the developer, and though there occurs a mismatch with an ideal latent image contrast shown by the environmental sensor, there is no mention made of a counter measure to meet such a mismatch.
Further, in Japanese Patent Application Laid-Open No. S63-177177, a proposal is made in which a hysteresis of the outputs of the detecting means in a plurality of timings within the past predetermined period of times is stored, and for example, in the case of the installation time of the developing apparatus into the image forming apparatus and the total replacement time of the developer inside the developing apparatus according to the stored plurality of data, a moisture absorption state of the developer is presumed according to the similar state of the data, and the image forming condition is controlled according to the moisture absorption state.
However, in an environmental fluctuation from beneath low moisture to beneath high moisture and the environmental fluctuation from beneath high moisture to beneath low moisture, there is a difference between an ejecting velocity of the moisture and the moisture absorption velocity in the developer, which is difficult to control.
Hence, in Japanese Patent Application Laid-Open No. 2002-6684, a proposal is made in which a plurality of patch images are formed, and the latent image contrast (Dmax control) as well as a γ-LUT (Dhalf control) are corrected.
However, in the above described control, there is a problem that no mention is made of means of guaranteeing a developer characteristic change based on a tribo change of the developer at the stage in which the developer is moisture conditioned.
Describing more in detail, in case the developing apparatus is transferred under low temperature low moisture environment (NL) in a moisture conditioned state under high temperature high moisture (HH) environment, at that point, the environmental sensor estimates that the tribo of the developer is high since the apparatus is under the NL, and determines that a high latent image contrast is required. Nevertheless, as described above, since the developer is not yet sufficiently moisture conditioned, the tribo is low, and the developing property is in a high state. Consequently, in the latent image contrast presumed from the environmental sensor, a situation occurs in which the density is outputted thick. Hence, by executing the Dmax control, the latent image contrast actually used is set to a low value again, so that the density can be set constant. However, when the developer is gradually moisture conditioned to the tribo under the NL, since the tribo rises up and the developing property lowers, an output is not made in the maximum image density by the Dmax control previously performed with a result that a replenishment excessive signal is transmitted. As a result, the TD ratio is increased and a fogging phenomenon sometimes occurs.
That is, instead of the latent image contrast becoming large because of becoming a high tribo under the NL, the tribo of the developer is in a low tribo state by the developer of the high toner density, and as a result, the control often ends up becoming inconsistent by being controlled in a low state of the latent image contrast.